The present invention relates to an electrostatographic apparatus such as an electrostatic copying machine comprising sensor means for sensing for the presence of a copy sheet at various locations along a sheet feed path and thereby sensing for a sheet jam.
In a typical electrostatic copying machine a copy sheet is fed through a sheet feed path for toner image transfer, fixing, discharge, etc. On occasion, a sheet will jam at some location along the sheet feed path and cause a malfunction of the feed system. It is desirable under these circumstances to shut down the copying machine as quickly as possible to limit the malfunction to a jam of only one sheet rather than several sheets which might actually damage the copying machine mechanism. It is also desirable to provide an indication of the location of the jam so that the jammed sheet may be removed quickly.
It has been practiced in the art to provide a plurality of sensors such as photosensors along the sheet feed path to sense for the presence of a copy sheet. The outputs of the sensors are connected through individual amplifiers and AND gates to an indicator means. A timing signal generator feeds timing pulses to the AND gates at times when the sheet should be present at the respective sensors, thus enabling the AND gates. If the sensor corresponding to an enabled AND gate produces a signal indicating absence of the sheet, this signal is gated through the AND gate to the indicator means to indicate a sheet jam. In other words, if a sensor produces an output indicating absence of the copy sheet at a time when the copy sheet should be present, it means that the sheet jammed or otherwise failed to be fed at a location upstream of the sensor. If the adjacent upstream sensor indicated presence of the sheet, it means that the sheet jammed between the adjacent upstream sensor and the sensor corresponding to the AND gate receiving the timing signal. This enables automatic indication of the location of the jam.
In a practical copying machine having a long sheet feed path, there may be provided on the order of 24 sensors along the sheet feed path. If a collation or sorting mechanism is provided, the number of sensors may be as high as 30 to 40. There will be further provided an amplifier and AND gate for each sensor. This results in an excessively large space requirement for the amplifiers and AND gates which are typically mounted on a printed circuit board and a complicated and expensive printed circuit board pattern. In addition, the power requirements for the amplifiers and sensors are unnecessarily high since only one or a few sensors are required to produce outputs at any one particular time.